Discussion
In this novel study we analyzed the impact of pancreatic, cardiac, and pituitary IO quantified in a cohort of pediatric cancer patients evaluated for transfusional IO as part of their clinical care. The results highlight the significant level of IO in some pediatric cancer patients with the majority of patients in this cohort showing at least moderate levels of IO ( LIC > 7 mg/g). Importantly, at least 80% had evidence of extrahepatic iron loading (pancreas, pituitary, heart) which implies significant exposure to the toxic, reactive ferrous form of iron.11,23 The high occurrence and magnitude of extrahepatic iron compared to a previous cohort randomly sampled across diagnoses1 reflects sample bias as subjects entered this cohort because their oncologists had enough clinical concern to obtain iron studies. Hence, the diagnoses in Table 1 are routinely treated with intense transfusion and chemotherapy.
In addition to iron loading through transferrin receptor-mediated processes and pathological loading via ion channels11, the liver loads iron by reticuloendothelial ingestion of erythrocytes. Thus, increase in LIC is linearly related to the number of transfusions and can be considered a surrogate for transfusion volume in the absence of chelation therapy or significant bleeding. In contrast, pathological extrahepatic iron loading occurs only when circulating reactive ferrous (Fe++) iron enters via divalent ion transporters (Zn++ and Ca++) which are not down-regulated by intracellular iron.11,24 The reactive ferrous sub species of non-transferrin-bound iron (NTBI) is referred to as labile plasma iron (LPI).
Normally, iron from senescent autologous or transfused RBC is recycled and used to make new RBC. However, when erythropoiesis is suppressed after cytotoxic chemotherapy, the iron binding ability of transferrin is exceeded and levels of the NTBI and ferrous iron become very elevated leading to unregulated transport of reactive iron into extrahepatic sites.3,25,26 Thus, erythropoietic activity is a major regulator of ferrous iron levels, iron toxicity, and distribution of iron into endocrine organs and heart. Iron is only detectable by MRI after it is converted to non-reactive ferric iron and stored as aggregates of ferritin.27-29 Organ dysfunction does correlate with the amount of extrahepatic iron detected by MRI.11,22,30,31
This physiology is supported by numerous studies of iron loading, extrahepatic iron distribution, end organ function and response to chelation in individuals with chronic transfusion dependent anemia associated with normal, ineffective, and no erythropoiesis.32,33 The liver loads first and independently of marrow activity, presumably because of the large reticuloendothelial space and direct ingestion of RBC.32 In the presence of high circulating NTBI/LPI, the pancreas loads sooner than the heart.19 Presumably the relative loading rates of the extrahepatic sites depends on differing kinetics of ferrous iron transport in various divalent iron transporters in different organs.
Patients treated for high-risk malignancies have often been exposed to intensive chemotherapy that can transiently shut off erythropoiesis, leading to prolonged exposure to elevated levels of NTBI/LPI due to the absence of erythropoiesis, with NTBI/LPI levels subsequently decreasing with marrow recovery as has been clearly shown during HSCT conditioning3,26. Transferrin saturation, an indirect measure of LPI,33 often increases to >80% in HSCT even when the patient is not iron loaded at the outset3,26 and is inversely related to reticulocyte count.25,34
Most subjects (80%) had some level of pancreatic IO, and 16% had levels in range that has been associated with glucose intolerance and diabetes.35 Twenty-eight percent (8/29) of the pituitary MRIs had siderosis and pancreatic siderosis was evident in all those evaluated, consistent with prior studies of pituitary iron loading in transfusion dependent anemia.22 Pituitary volume loss has been associated with pituitary dysfunction in transfused populations even in the absence of pituitary siderosis,22 suggesting that LPI may cause damage even before there is enough loading to be detected by MRI. However, in this cohort, cranial radiation or chemotherapy likely affect pituitary volume and function as well. Theoretically, oxidant damage from radiation or chemotherapy will be markedly amplified by the presence of ferrous iron. This is suggested by the 29-fold increased risk of second malignancy when radiation is delivered with myelosuppressive chemotherapy.36 Thus, we cannot exclude a possible contribution of LPI to the volume loss seen in the brain tumor patients even though there was no detectable iron in the pituitary (Figure 1). This raises interesting questions regarding treatment timing and delivery of radiation in proximity to marrow suppression when reactive iron levels would be very high. These pituitary iron and volume changes are particularly thought provoking considering that over 75% of survivors of childhood cancer have pituitary dysfunction by 50 years of age.37
Not surprisingly, there was no relation between treatment intensity (ITR-3)13 and magnitude of IO or distribution of iron loading in this cohort. This was likely due to insufficient variability among the subjects as all were in the highest levels of the intensity scale; further, this scale does not specifically consider magnitude and duration of erythroid suppression. HSCT also did not influence iron loading or distribution in this cohort, again likely due to the overall higher intensity of treatment received by this cohort. Prior studies indicate higher-intensity therapy, particularly HSCT, is associated with higher transfusion burden and higher levels of NTBI, thereby increasing risk of IO in both hepatic and extrahepatic sites.38-40
While ferritin levels were obtained in this study and were often used by the referring oncologists to screen for iron loading, the diagnosis of IO was based on MRI determination of LIC. Ferritin levels do correlate with LIC in large populations but the variability is very large making single measurements not useful and even trends can be misleading with ferritin trending upward when LIC is stable or dropping.41 We used ferritin trends to optimize timing of confirmatory MRI for assessment of tissue iron as this is the “gold standard”, although ferritin can be used to infer iron burden in resource restricted settings. We found ferritin over 800 ng/dl is 80% predictive of a LIC >3 mg/g, consistent with data from large populations of transfusion dependent anemia patients,42-44 and confirmation with repeated measurement should prompt MRI for verification of IO to more accurately assess if iron removing therapy is needed. Transferrin saturation is also highly variable, although in large epidemiological studies, elevated transferrin alone predicts early death and higher probability of malignancy.45 An elevated iron saturation, >60% on several measures, indicates higher likelihood of exposure to LPI, which increases risk of endocrine and cardiac iron deposition, and utilization of chelation instead of or in addition to phlebotomy should be considered. In fact, high pancreatic iron likely prompted a number of subjects in this cohort to be treated with chelation rather than phlebotomy (Figure 3B).
The primary and novel conclusion of this work is that there is significant partitioning of iron into extrahepatic sites in a large percent of pediatric patients after aggressive treatment for cancer. This extrahepatic loading only occurs with significant exposure to the highly reactive and toxic ferrous form of iron (LPI). The presence of elevated LPI is caused by decreased marrow erythroid activity in the face of multiple transfusions.
Strengths of this study include a diverse patient population with regard to age and diagnosis, and the inclusion of initial and follow up MRI evaluation of liver, pancreas, cardiac and pituitary, the last of which is novel in this setting. Inclusion of all referred patients and consistency in diagnostic approach are additional strengths. Limitations include that this was an at-risk cohort specifically evaluated over concern for possible IO. Thus, these results are generalizable to children treated with higher-intensity cancer therapy and many PRBC transfusions, but less so to childhood cancer patients as a whole. An additional limitation was the inability to include PRBC transfusion data due to variations in documentation over the study period.
The goal of this work is to protect cancer survivors from the toxicity of iron over their lifespan. It is clear from work outside the oncology realm that iron toxicity is primarily related to the amount of tissue ferrous iron and the duration of exposure.23 This means a small amount of iron over many decades can cause damage. As an example, ineffective erythropoiesis with transfusion every three weeks and no chelation in thalassemia leads to pituitary dysfunction in about 5 years, diabetes in about 10, heart failure and death in about 15 years and increased cancer risk in the fourth and fifth decade.35,46,47 It is also clear that reducing iron can reverse endocrine failure and reduce risk of new cancers by 30% even when iron is treated in the sixth decade.10,48
There are many interesting questions raised by this data regarding IO and its incidence, prevalence, and relation to specific therapy in malignancy. Critically, we need to know how much iron is in the body after chemotherapy, and whether extrahepatic iron distribution is present. This information can be obtained by a single abdominal MRI with pancreas in the field. If the pancreas is positive, the heart and pituitary should be assessed. Since we do not know how much area-under-the-curve exposure to ferrous iron in childhood will cause issues decades later, it seems prudent to eliminate measurable pathologic iron as soon as is practical. Furthermore, measurable toxicity occurs long after loading is detected and likely after the patient leaves the pediatric environment. We think it is optimal that pathological iron be addressed before the patient transitions from the pediatric cancer treatment center. In addition to the magnitude and distribution of loading, family understanding of the reasons for iron removal, the likelihood that normalizing iron will reduce risk, reassurance of the certainty of our ability to safely remove the measurable iron, and design of a flexible approach that is acceptable to the family are all critical considerations.
Conflict of Interest Statement: There is no conflict of interest to disclose.
Acknowledgements: Nathan Smith, project manager at the Children’s Hospital cancer and blood institute for his contribution with data collection and organization and keeping the project on track.